Display device, electric device comprising such a display device and method for driving a display device

ABSTRACT

The invention relates to a display device and a method of driving a display device, wherein the display comprises a plurality of light emitting elements and means for applying a driving signal to the light emitting elements. The display device comprises control means for adjusting a duty cycle and a magnitude of the driving signal for at least one of the light emitting elements. In this way the uniformity of the display can be controlled while keeping the brightness of the display constant. Control of the uniformity of the display may e.g. be used in relation to control of the power consumption of the display device or improving the quality of dark images.

The invention relates to a display device comprising a display having aplurality of light emitting elements and means for applying a drivingsignal to said light emitting elements.

Display devices employing light emitting elements on or over a substrateare becoming increasingly popular. These light emitting elements may belight emitting diodes (LED's), incorporated in or forming display pixelsthat are arranged in a matrix of rows and columns. The materialsemployed in such LED's are suitable to generate light if a current isconveyed through these materials, such as particular polymeric (PLED) ororganic (OLED) materials. Accordingly the LED's have to be arranged suchthat a flow of current can be driven through these light emittingmaterials. Typically passively and actively driven matrix display aredistinguished. For active matrix displays, the display pixels themselvescomprise active circuitry such as one or more transistors.

In the usual manner of driving an active matrix display, all pixels emitlight continuously when addressed. This state is referred to as a 100%duty cycle, wherein the duty cycle is defined as the percentage of timeduring which the display, or a light emitting element thereof, provideslight in a frame period. This method of driving has the disadvantagethat a low average current passes the drive transistors of the displaypixels, which has a negative effect on the display uniformity.Uniformity is defined as the variation in brightness level between thedifferent light emitting elements when driven with a driving current ofequal magnitude. In addition, the display suffers from sample/holdeffects that may blur e.g. video images. Sample/hold effects arise fromthe fact that in every frame period, a new image may be displayed at thestart of the frame period (sample), whilst in remainder of the frameperiod (typically 16 msec for 60 Hz operation) the image remains visibleon the screen (hold). For moving video images, the eye tries to followthe image across the display, whilst, due to the sample/hold nature ofthe addressing, the image is physically stationary. The user interpretsthis effect as a blurred image.

A method for avoiding these problems is to drive the active matrixdisplay in a pulsed mode, wherein the display or the light emittingelements only emit light for a fraction of the time in the frame period,i.e. a reduced duty cycle. However, such an active matrix display,driven in a pulsed mode, gives rise to an increase in power consumption.

It is an object of the invention to provide an improved display deviceeliminating or reducing at least one of the above-mentioneddisadvantages.

This object is achieved by employing a display device where controlmeans are provided adapted to adjust a duty cycle and a magnitude ofsaid driving signal for at least one of said light emitting elements. Byadapting the duty cycle and in accordance therewith the magnitude of thedriving signal or vice versa, the uniformity of the display or displaypixels can be adjusted. It is noted that generally the product of dutycycle for and current conveyed by the light emitting element issubstantially constant, as a result of which the variation of thebrightness levels at a particular driving signal between different lightemitting elements can be adjusted, while maintaining the averageperceived brightness of the light emitting pixels at the original level.

In an embodiment of the invention the control means are adapted toselect a single mode out of a plurality of available modes with respectto the uniformity of the display or display pixels. One advantage isthat in choosing a particular mode with respect to the uniformity, thepower consumption of the display device can be influenced. Anotheradvantage relates to the flexibility in adapting the quality of theimage on the display.

In an embodiment of the invention the display device comprises selectionmeans for selecting one of the available modes by a user. The user ofthe display device may adapt the uniformity of the image if he sodesires.

In an embodiment of the invention the single mode with respect touniformity is selected in accordance with the power available orremaining for an electric device comprising the display device. Anadvantage of this embodiment is that the display device mayautomatically switch to a lower uniformity for the display, if the powerfor the device falls below a certain level, thereby increasing the timeduring which the display device can be used.

In a preferred embodiment of the invention the single mode is selectedin response to the data to be displayed on the display and/or receivedby said display device or electric device. This provides the possibilitythat the uniformity of the display is automatically adjusted dependingon whether the display is actively used or in a so-called stand by mode.Moreover the uniformity of the display and/or the power consumption canbe adjusted automatically if the data to be displayed gives rise to suchan adjustment, e.g. if the image to be displayed is on average dark. Inaddition the number of grey levels, i.e. visible brightness levels isdynamically increased if uniformity is increased and the duty cycle isreduced for such dark images.

In a preferred embodiment the single mode is selected in accordance withthe rate of change of the data to be displayed on the display. Thisprovides the advantage that for moving images to be displayed, theuniformity can be increased automatically by increasing the grey level.In addition sample/hold artefacts may be avoided in this embodiment,since a shorter duty cycle, which reduces the hold period, results in aperceived sharper image of moving objects.

It should be clear that for the embodiments presented above that thesingle mode may be selected by the user or automatically and dynamicallyfrom the available modes. As a result the functionality of the displaydevice is enhanced.

In the embodiments discussed above, it was assumed that the entiredisplay operated in the same mode, i.e. the same uniformity for theentire display. However, in a preferred embodiment the display comprisesat least a first part displayed in a first mode of said available modesand a second part displayed in a second mode of said available modes.This has the advantage that if e.g. different images are to be displayedon different parts on the display, different modes with respect touniformity can be employed.

It should be appreciated that the embodiments, or aspects thereof, maybe combined.

The invention further relates to an electric device comprising a displaydevice as described in the previous paragraphs. Such an electric devicemay relate to handheld devices such as a mobile phone, a PersonalDigital Assistant (PDA) or a portable computer as well as to devicessuch as a Personal Computer, a television set or a display on e.g. adashboard of a car. It is noted that the issue of power consumption isparticularly relevant for battery powered devices.

The invention further relates to a method for driving a display by adriving signal, said display having a plurality of light emittingelements comprising the step of adjusting a duty cycle and magnitude ofsaid driving signal in accordance with each other for at least one ofsaid light emitting elements. It is noted that this method is not onlyapplicable to PLED or OLED devices, but more generally to deviceswherein the light intensity is defined by the current delivered by adriving transistor of which the characteristics may vary from onetransistor to another. Examples include electroluminescent displaydevices, active matrix display devices bases on field emissiontechniques and electrochromic or switching mirror type of displaydevices.

It is noted that WO 02/27700 discloses a display device comprising adriver circuit which modulates the duty cycle of the on-state of a pixelduring a frame period. However, in this publication the duty cycle isadjusted in order to obtain a particular pixel brightness using pulsewidth modulation, without changing the magnitude of the driving signal.Uniformity of the display is not an issue in this publication.

U.S. 2002/084463 discloses a CMOS driving circuit for an OLED display,wherein the luminance is controlled by application of a duty factor.Again the driving is performed by digital pulse width modulation, so nomeans for changing the brightness of the pixels are disclosed. Inaddition, for CMOS driven display devices uniformity of the display isgenerally not an issue, in contrast to display devices applyingpoly-silicon (p-Si) or amorphous-silicon (a-Si) driving transistors ofwhich the characteristics may vary considerably from one transistor toanother. Standard CMOS-drivers are usually applied for micro-displaysand cannot easily handle high voltages.

The invention will be further illustrated with reference to the attacheddrawing, which shows preferred embodiments according to the invention.It will be understood that the device and method according to theinvention are not in any way restricted to this specific and preferredembodiment.

FIG. 1 shows an electric device according to an embodiment of theinvention;

FIG. 2 shows a first arrangement for an active matrix display accordingto an embodiment of the invention;

FIGS. 3A and B show an embodiment of a display pixel for a voltageaddressed active matrix display and the behaviour of the brightnessvariation at various grey levels for the display pixels;

FIGS. 4A and B show an embodiment of a display pixel for a currentaddressed active matrix display and the behaviour of the brightnessvariation at various grey levels for the display pixels;

FIG. 5 shows an embodiment of a display pixel for an active matrixdisplay illustrating various alternatives for adjusting the duty cycleof the display pixel;

FIG. 6 shows a second arrangement for an active matrix display accordingto an embodiment of the invention.

FIG. 1 shows an electric device 1 comprising a display 2 having aplurality of display pixels 3 arranged in a matrix of rows and columns.Display 2 may comprise one or more parts 4, 5 that appear on the display2 as windows or pop-up screens for displaying different kinds or typesof information or data compared to the main display 2. Part 4 may e.g.show a menu facility that is prompted via a remote control (not shown).The menu facility may provide the user of the device 1 with options ofadjusting e.g. the brightness and/or the contrast of the display 2.According to an embodiment of the invention this menu facility may alsoinclude an option for adjusting the uniformity of the display 2 ordisplay pixels 3 or display part 5. Alternatively or in addition theelectric device 1 may comprise a control dial or button 6 that may beemployed by the user to adjust the uniformity of the display 2 ordisplay parts 4, 5. The display parts 4, 5 of different uniformity maybe present or called by the user in a single display. The multipleuniformities in one display 2 can be achieved by operating the variousparts 4, 5 of the display 2 at different duty cycles. It should beappreciated that the remainder of the display 2 may operate in a thirdmode having a different uniformity than parts 4, 5. Examples of suchapplications include windows in multimedia applications orpicture-in-picture (PIP) for television screens, wherein e.g. videoimage sections are subject to a lower duty cycle, while stationary imagesections operate at a higher duty cycle. Another example relates tomobile phones, wherein a first part 4 of the display 2 may be in astand-by state and a second part 5 of the display 2 is actively used. Itis noted that in general the parts 4, 5 of the display 2 operating in aparticular single mode do not have to be pre-defined, but may vary inlocation on the display 2 from frame to frame as defined by the controlmeans (see FIG. 2).

FIG. 2 shows a display device 7, comprising the display 2 of theelectric device 1 as shown in FIG. 1. The display 2 comprises a rowselection circuit 8 and a data register 9. Information or data, such as(video)images, received via line 10 and to be presented on the display 2is input to the control unit 11 which information or data issubsequently transmitted by the control unit 11 to the appropriate partsof the data register 9 via line 12. The selection of the rows of displaypixels 3 is performed by the row selection circuit 8 via line 13. Dataare written to the display pixels 3 from the data register 9 via line14.

FIG. 3A shows a known arrangement for a display pixel 3 comprising anaddressing transistor T1, a storage capacitor C and means T2 forapplying a driving signal to a light emitting element 15. T2 may be ap-Si thin film transistor (TFT) and light emitting element 15 may be alight emitting diode, such as a PLED or an OLED. One of the plates ofthe capacitor C and the source electrode of T2 are connected to avoltage supply line 16.

If T2 is biased in saturation it behaves as a constant current source,passing a current which is proportional to μ_(fe). (V_(GS)−V_(T))² whereV_(GS) is the gate-source voltage of T2, V_(T) the threshold voltage,and life is the field effect mobility of T2. This constant current isthen driven through the LED 15 which is connected to T2. Thus, thecurrent source is programmed by setting the voltage on the gate of T2.This is achieved during a short addressing time of e.g. 25 μs by turningon T1 via line 13 and transferring the signal voltage from the dataregister 9 to the gate of T2. T1 is then switched off, and theprogrammed voltage is held on the gate of T2 for the rest of the frametime. The storage capacitor C prevents appreciable discharge of thisnode via leakage through T1, thus forming a memory to allow continuousLED current while the other rows of the display 2 are selectedsequentially. This addressing scheme works well, but requires very highuniformity in the characteristics of T2 for substantially each displaypixel 3 in the display 2, since the current is proportional both to(V_(GS)−V_(T))² and to μ_(fe). The circuit is also prone to some secondorder horizontal cross-talk effects. These arise because there is acurrent flowing through T2 and the LED 15 during the addressing period,and because the current carrying row electrodes have a finiteresistance. Thus, there are voltage drops along the current carryingrow, the source voltage of T2 is no longer well defined, and so thevalues of V_(GS) are in error. In the arrangement shown in FIG. 3A, ann-channel transistor (T3) is added in series with the current source T2and the PLED 15. This transistor T3 switches off the current flow duringthe addressing period, which reduces the voltage programming errordescribed above.

For drive transistors T2, variations for μ_(fe) and V_(T) in the rangeof 5-10% are typically observed. FIG. 3B shows a simulation result for adisplay 2 comprising display pixels 3 as depicted in FIG. 3A, whereinthe behavior of the brightness variation BV between different displaypixels 3 as a percentage of the grey level GL was obtained for variousgrey levels of the LED 15. Grey level or brightness level is a measurefor the amount of current conveyed by the LED 15, however, notnecessarily in a linear relation. It is clear from the simulation resultpresented in FIG. 3B that a significant variation of the brightnessbetween different display pixels 3 may arise, especially for variationsfor μ_(fe) and V_(T) in the range of 5-10%. As an example at a greylevel of 4, which corresponds with a particular current magnitude, thebrightness of a LED 15 may be 80% higher than for an adjacent LED 15,while driven with the same current magnitude, (see dashed line presumedthat the characteristics of the drive transistors T2 for the LEDs 15vary in the range of 10%. It is noted that the brightness variation BVbetween different display pixels 3 decreases with increasing grey level,i.e. if the LEDs 15 convey higher currents, i.e. higher magnitude of thedriving signal.

A current mirror pixel circuit as shown in FIG. 4A may reduce theeffects resulting from the variation in the characteristics for T2,while still operating in an analogue mode. The driving transistor T2 isused in both addressing the display pixel 3 and in driving the LED 15.The data input signal is applied as a current rather than a voltage overthe line 14, indicated by the current source I. During the addressingperiod the driving transistor T2 is diode-connected by the transistor T4via addressing transistor T1, and the LED 15 is isolated from thecircuit by the transistor T3. During this addressing period the datainput current is forced through T2 while the capacitor C is charged toreach the associated gate-source voltage V_(GS) for T2. Now, by openingT1 and T4 and by closing T3, the drain current is fed to the LED 15. Thememory function of the capacitor C assures the LED current to be aperfect copy of the data input current received over line 14.

This description corresponds to an ideal circuit operation for thedisplay pixel 3 as shown in FIG. 4A. In practice, issues e.g. relatingto differences between the driving transistor T2 drain-source voltageduring addressing and driving will give rise to errors, such that thedriving current still has some dependency on the characteristics of theindividual transistor T2 and LED 15. However, this dependency turns outto be much smaller than in the case of the current source pixel circuit.The main advantage of the current mirror circuit is the reducedinfluence of V_(T) and μ_(fe) spread. FIG. 4B shows the calculatedbrightness variation. Compared to FIG. 3B, approximately one order ofmagnitude improvement in the brightness variation BV over the display 2is observed, however display pixels 3 conveying higher currents stillare more uniform in brightness.

It is the gist of the invention that use is made of the observedbehaviour of the brightness variation BV with the grey value GL of alight emitting element 15. By adjusting the magnitude of the drivingsignal, a mode with respect to a desired or adequate uniformity can beselected corresponding to a point on the curves of FIG. 3B or FIG. 4B.In fact the curves represent available modes with respect to uniformity,out of which one or more single modes can be selected or are selectedthat are appropriate for the situation. It is noted that the drivingsignal may be a current with a particular magnitude, as discussed above,but may also be a voltage signal of a certain magnitude giving rise to acurrent with a magnitude determined by the light emitting elementitself. This voltage signal is e.g. achieved if T2 acts as an openswitch. By adjusting the duty cycle in accordance with the magnitude ofthe current conveyed by the light emitting elements, e.g. powerconsumption and image quality can be controlled manually orautomatically.

FIG. 5 shows several ways in which the duty cycle can be adjusted for avoltage addressed active matrix driving scheme. One way to adjust theduty cycle is by applying an appropriate reverse voltage for a certainpercentage of time of the frame period for the LED 15 of the displaypixel 3, indicated by the voltage source 17. If the voltage source 17prevents current to be conveyed by the LED 15 for e.g. 20% of the frameperiod a duty cycle of 80% is obtained. By setting the appropriate timeduring which the reverse voltage is applied by the voltage source 17 toe.g. all the display pixels 3 of the display 2, the required duty cyclecan be obtained for the entire display 2. In a similar manner, the powerline voltage 16 can be made adjustable to define the duty cycle.

Alternatively a switch T5, such as a power transistor, can be appliedpreventing that current is conveyed by the LED 15. The switch T5 can beaddressed over a duty cycle select line 18 that is controlled by thecontrol unit 11. By appropriate addressing of the duty cycle via thecontrol unit 11, different duty cycles can be obtained for differentparts 4, 5 of the display 2.

In yet another alternative additional addressing pulses can beincorporated into a frame period (e.g. the display 2 may be addressedtwo or more times during a frame instead of once). In this way,sub-frames are created. By addressing the display 2, or parts 4, 5 ofthe display 2, with a grey level associated with a black pixel for someof the sub-frames it is possible to adjust the duty cycle for thedisplay 2.

It is noted that various other ways of adjusting the duty cycle areknown. The invention does not rely on the way in which the duty cyclecan be varied.

The selection of a mode with respect to the uniformity of the display 2may be performed by (automatically) adjusting the duty cycle of thedisplay 2. If e.g. the duty cycle is decreased, the magnitude of thedriving signal, i.e. the current for the display pixel 3, may beincreased automatically by the control means 11 such that the perceivedaverage brightness of the display 2 or display pixels 3 remainsconstant. The increase in the magnitude of the current has two effects.A shift on the curves to a higher grey level as illustrated in FIGS. 3Band 4B is obtained, as a result of which the uniformity of the display 2is increased. Moreover, since the current magnitude is increased thepower consumption for the LED 15 generally increases as well. By thismechanism, a display 2 with e.g. two available modes can be envisaged,one mode relating to low power consumption and low uniformity for thedisplay and the other mode relating to high power consumption and highuniformity for the display. In general a duty cycle that can becontinuously adjusted in the range of e.g. 1-100% will result in anunlimited number of available modes which trade off uniformity forpower. These modes can be chosen by the user in several ways, some ofwhich were already discussed for FIG. 1.

With regard to the relation between uniformity of the display 2 and thepower consumption, a display device 7 may e.g. operate by default in thehigh uniformity mode, corresponding to a low duty cycle and high powerconsumption. However, if the battery power falls below a certain level,that may be user defined, the display device 7 may switch, e.g.initiated by the control means 11, to a low uniformity mode, as a resultof which power consumption is reduced. This has the advantage that thedisplay device 7, especially when implemented in a battery poweredelectric device 1, may be used for a longer period before the device 1is out of power.

The uniformity mode may alternatively or in addition relate to theoperation state of the display 2. If the display 2 is e.g. in a standbystate, the uniformity of the display 2 may be low as a result of whichpower consumption is reduced. If the display 2 switches to an activestate, the display device may switch to another single mode relating toan increased uniformity for the display 2, by decreasing the duty cycleand increasing the current thought the light emitting elements, if thecontrol means 11 is triggered with respect to the active state of thedisplay 2.

The mode for the uniformity of the display 2 may be automaticallyselected in response to the type or content of the data, received by thecontrol means 11 over line 10. If the image to be displayed is onaverage bright, it may be preferred to have a mode selected by thecontrol means 11, wherein the duty cycle is increased, as the display 2has already a reasonable uniformity. As a result power can be saved ifsuch data are presented. However, if the image to be displayed is onaverage dark, a mode may be preferred wherein the uniformity of thedisplay 2 is increased. This mode is selected by reducing the duty cycleand increasing the magnitude of the driving signal, e.g. by the controlmeans 11. In this way, the duty cycle also dynamically adjusts theaverage brightness of the image. In addition, this reduced duty cycleincreases the number of grey levels which can be made visible in thedark image, whilst maintaining the average brightness of the image to bedisplayed. If e.g. the duty cycle is decreased to 10%, the inventionallows dividing the range of perceived brightness levels for the darkimage in ten times smaller sections, if data containing these extrabrightness levels is available. In this way more grey levels can becreated in the dark image, thus the quality of the image can besignificantly improved. In addition the selected single mode may relateto the quality of the data, e.g. with respect to the coding format (forexample MPEG coding), to be displayed.

In general, if moving images are to be displayed, the uniformity of thedisplay 2 should be increased. This feature can be implemented by havingthe control means 11 detecting the rate of change of the data to bedisplayed and adjusting the duty cycle and magnitude of the drivingsignal in accordance with the rate of change such that uniformity isincreased.

As was discussed for FIG. 1, the display 2 may have several parts 4, 5for which a different mode with respect to uniformity of such a part 4,5 can be selected. This can e.g. be achieved by transmitting differentappropriate signals from the control means 11 over the duty cycle selectline 18 to the switches T5 for the display pixels 3 constituting theparts 4 and 5. For example, part 5 may be a pop-up window showing avideo on a display 2 of a computer monitor 1. Control means 11 detectsthe video data received over line 10 and instructs the display pixels 3constituting the display part 5 to be driven at a lower duty cycle viaduty cycle select line 18 and with a higher magnitude for the drivingsignal. In this way the uniformity of the part 5 is enhanced, while theremainder of the display 2 operates in a lower uniformity mode.

FIG. 6 shows a schematic illustration of a display device 7 adapted toperform the functions as described above. The control means 11 isadapted to control the duty cycle of the display 2 or the display pixels3, e.g. via duty cycle select line 18. This duty cycle can e.g. beadjusted by a user via control button 6. As described above, the dutycycle can be varied in other ways as well, e.g. by analysing the datareceived over line 10. The control means 11 is adapted to adjust themagnitude of the driving signal to be sent over line 14 in accordancewith the adjusted duty cycle. It is noted that while in general theproduct of duty cycle and current conveyed by the light emitting elementmay be substantially constant, it is not excluded that both the dutycycle and current through the light emitting elements are decreased orincreased for certain applications.

1. Display device comprising a display having a plurality of light emitting elements and means for applying a driving signal to said light emitting elements, wherein control means are provided adapted to adjust a duty cycle and a magnitude of said driving signal for at least one of said light emitting elements, and wherein said control means are adapted to select a single mode out of a plurality of available modes with respect to uniformity of said display or said light emitting elements.
 2. Display device according to claim 1, wherein said display is an active matrix emissive display comprising light emitting diodes and said means for applying a driving signal are driving transistors, each driving transistor being associated with one of said light emitting diodes.
 3. Display device according to claim 1, wherein said display device comprises selection means, such as a switch, dial or menu facility, for selecting one of said available modes by a user of said display device.
 4. Display device according to claim 1, wherein said single mode is selected in accordance with the available or remaining power for an electric device comprising said display device.
 5. Display device according to claim 1, wherein said single mode is selected in response to data to be displayed on said display and/or received by said device.
 6. Display device according to claim 5, wherein said duty cycle dynamically adjusts an average driving signal for said display.
 7. Display device according to claim 5, wherein said single mode is selected in accordance with the rate of change of data to be displayed on said display.
 8. Display device according to claim 1, wherein said control means are adapted to select at least a first mode of said available modes for a first part of said display and a second mode of said available modes for a second part of said display.
 9. Electric device comprising a display device according to claim
 1. 10. Method for driving a display by a driving signal, said display having a plurality of light emitting elements, comprising adjusting a duty cycle and magnitude of said driving signal in accordance with each other for at least one of said light emitting elements, and selecting a single mode out of a plurality of available modes with respect to uniformity of said display or said light emitting elements.
 11. The method of claim 10, further comprising: selecting the single mode in accordance with the available or remaining Power for an electric device comprising said display device.
 12. The method of claim 10, further comprising: selecting the single mode in response to data to be displayed on said display and/or received by said device.
 13. The method of claim 12, further comprising: selecting the single mode in accordance with the rate of change of data to be displayed on said display. 